Fuel cell apparatus and methods

ABSTRACT

Fuel cells having an efficient means of thermal insulation such that all of the components requiring high temperature operation are contained within a single housing and whereby such thermal insulation is disposed exterior to such housing.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application claims priority to provisional application Ser. No.60/682187, filed May 18, 2005, entitled Fuel Cell Apparatus and Methods,the contents of which are incorporated herein.

TECHNICAL FIELD

The invention relates to apparatus and methods that improve fuel cellefficiency and safety. In one embodiment, the invention relates to fuelcells adapted for improving energy balance by integrating multiple fuelcell components in an isothermal zone.

BACKGROUND

Fuel cells that operate in conjunction with replaceable fuel canistersfilled with, for example, gaseous hydrogen, methanol, butane or dieselfuel, are a developing technology. These types of fuel cells aredesigned to compete with the various battery solutions that powerconsumer products. The competitiveness of these fuel cells with regardto batteries depends on a number of factors, such as the energy densityof the fuel in the canister; the ability of the fuel cell to convertchemical energy to electrical energy with certain efficiencies; and theneed to keep the fuel cell stack, along with associated fluid pumpingand power control components, no larger than that of a competitivebattery.

Improvements in energy density and chemical conversion efficiency havebeen achieved with solid-oxide-fuel cells (SOFCs), which utilize ceramicmembranes instead of polymer membranes. Because solid-oxide fuel cellscan convert a variety of different molecular fuel types intoelectricity, e.g., various hydrocarbons, a solid-oxide fuel cell canutilize energy dense liquid fuels and still achieve suitable energyconversion efficiencies.

However, solid-oxide fuel cells, require membrane and catalyticoperation at temperatures in excess of 600° C., often in excess of 750°C. Consequently, designers of solid-oxide fuel cells for portable powerapplications must protect the end user from the extreme heat withoutadding excessively to the size of the overall system. Additionally, apresent day solid-oxide fuel cell operating at 800° C. can easilyradiate or transmit ten times more energy to the environment as wasteheat than the electrical energy delivered to the user. Such a systemcannot be more than 10% efficient, i.e., the system uses more than 90%of the fuel energy for the sole purpose of maintaining the reactor's800° C. operating temperature. Therefore, with such low efficiency, itis unlikely for current solid-oxide fuel cells to compete withbatteries.

State-of-the-art portable solid-oxide fuel cells have not been able toachieve similar volumes to batteries. Solid-oxide fuel cell generator,without insulation, rarely exceeds 0.35 watts per cubic centimeter(W/cc). Upon adding insulating layers with thickness sufficient forenergy efficient operation, most conventional solid-oxide fuel cellsprovide power to volume ratios below 0.1 W/cc.

Additionally, existing fuel cell apparatus and systems designs provideheated components (other than the solid-oxide fuel cell stack) toimprove the efficiency of the system. However, each heated componentadds to the volume of the apparatus and to the amount of insulationrequired to avoid excessive heat dissipation.

As a result, there exists a need to build a miniature fuel cellapparatus, which when combined with a portable fuel canister, canprovide energy storage capacities similar to or exceeding that ofrechargeable batteries, e.g., greater than 200 Watt-hours per liter(W-hr/L), and preferably greater than 400 W-hr/L. A fuel cell would beof great value for powering portable electronics, whose functions todayare often limited by the energy capacity of batteries. In addition,given the many potential power supply applications of interest toindividual consumers, a fuel cell that is safe for individual users isalso of great value.

SUMMARY OF THE INVENTION

Fuel efficiency is achieved, in part, by regulating thermal efficiency.Specifically, improved thermal efficiency results, in part, from any oneor a combination of the following factors: integration of the fuelreformer, fuel cell, and tail gas burner into a single, essentiallyisothermal, zone of high temperature; substantially reducing the heatdissipation area of the hot zone by increasing the power density in thefuel cell stack, preferably to values in excess of 2 W/cc; use of anefficient (either aerogel or vacuum) means of thermal insulation suchthat all of the components requiring high temperature operation arecontained within a single housing and whereby such thermal insulation isdisposed exterior to such housing; incorporation of low-thermalconductance connections for exchanging fluids between the fuel cellapparatus and the outside world and for the extraction of electricalcurrents from the fuel cell; and/or incorporation of a heat recuperator,preferably located within the thermal insulation zone, such that theheat recuperator can operate at a temperature intermediate between thetemperature of the hot zone and the outside ambient.

As used herein, “fuel cell apparatus” and “fuel cell systems” refer toan apparatus or device that can contain some or all of the followingcomponents: a fuel reformer, a tail gas burner,anode/electrolyte/cathode elements, pumps, and controls. However, “fuelcell” refers to the anode/electrolyte/cathode membrane structure. Inaddition, “power density” refers to a ratio of the power generated in agiven volume and as otherwise understood in the fuel cell art.

Although, the invention relates to different aspects and embodiments, itis understood that the different aspects and embodiments disclosedherein can be integrated together as a whole or in part, as appropriate.Thus, each embodiment disclosed herein can be incorporated in each ofthe aspects to varying degrees as appropriate for a givenimplementation. Furthermore, although some aspects and embodiments aredescribed using “means for” terminology, it is understood that allaspects, embodiments, and other concepts disclosed herein can serve assupport for means plus function claims, even if specific “means for”language is not used in a specific portion of the written description.

It should be understood that the terms “a ,” “an,” and “the” mean “oneor more,” unless expressly specified otherwise.

As used herein “communication with” refers to direct or indirectcommunication, e.g., direct or indirect contact such as throughappropriate connections such as walls, tubes, semiconductor traces andlayers, wire, and other means as known in the art, and combinationsthereof.

In one aspect, the invention relates to a fuel cell apparatus thatincludes a housing and one or more safety features.

The foregoing, and other features and advantages of the invention, aswell as the invention itself, will be more fully understood from thedescription, drawings, and claims which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference to the figures herein is intended to provide a betterunderstanding of the methods and apparatus of the invention but are notintended to limit the scope of the invention to the specificallydepicted embodiments. The drawings are not necessarily to scale,emphasis instead being placed upon illustrating the principles of theinvention. Like reference characters in the respective figures typicallyindicate corresponding parts.

FIG. 1 is a cross-sectional side view of a fuel cell apparatus accordingto an illustrative embodiment of the invention;

FIG. 2 is a perspective view of a fuel cell apparatus component havingfluidic connections and a heat recuperator according to an illustrativeembodiment of the invention;

FIG. 3 is a schematic drawing of anodes, cathodes, and electrolytesarranged in a configuration suitable for use in the fuel cell apparatusaccording to an illustrative embodiment of the invention;

FIG. 4 is a cross-sectional side view of another fuel cell apparatusaccording to an illustrative embodiment of the invention;

FIG. 5 is a schematic drawing of flow routing layer suitable for usewith a fuel cell apparatus according to an illustrative embodiment ofthe invention;

FIG. 6 is a schematic diagram of a fuel cell apparatus incorporatingvarious safety features according to an illustrative embodiment of theinvention;

FIGS. 7A, 7B and 7C are schematic drawings illustrating control flowsfor regulating a fuel cell apparatus according to an illustrativeembodiment of the invention; and

FIG. 8 is a graph of butane conversion as a function of temperaturesuitable for setting temperature presets and reaction temperature rangesto regulate an embodiment of the invention.

DETAILED DESCRIPTION

The following description refers to the accompanying drawings thatillustrate certain embodiments of the present invention. Otherembodiments are possible and modifications may be made to theembodiments without departing from the spirit and scope of theinvention. Therefore, the following detailed description is not meant tolimit the present invention. Rather, the scope of the present inventionis defined by the appended claims.

It should be understood that the order of the steps of the methods ofthe invention is immaterial so long as the invention remains operable.Moreover, two or more steps may be conducted simultaneously unlessotherwise specified.

Integrated Fuel Cell Apparatus, Package, and Connections

The fuel cell apparatus embodiments described herein can produceelectrical power in excess of 2 W/cc and in excess of 3 W/cc. Such fuelcell apparatus are uniquely capable of producing insulated package sizessmall enough for portable application, even though the power ratings arebelow 100 watts, below 20 watts, or below 5 watts. In contrast, existingfuel cell designs only generate power densities typically below 0.5W/cc. As a result, the low power density fuel cells are too large andnot efficient enough for many applications such as, for example,consumer battery substitutes.

The power density (W/cc) depends primarily upon the design of theintegrated fuel cell apparatus and the individual fuel cell or fuel cellstack (plurality of individual fuel cells). In particular, the level ofproximal integration of the various fuel cell apparatus componentswithin the housing is an important design factor. As a result, fuel cellapparatus efficiency can be a function of how close the various fuelcell membranes can be placed subject to the constraints of mechanicalstrength and fluid routing. Many of the aspects and embodimentsdescribed herein relate to component integration within one thermalregion and related techniques to control thermal losses. The use ofsemiconductor structures in many of the embodiments disclosed hereinenables the small sizes and high energy densities that allow for fuelcell apparatus that can compete with various battery types.

FIG. 1 shows one example of a fuel cell apparatus 5, in cross-sectionalview. FIG. 1 illustrates a fuel reformer 10, a pair of solid oxide fuelcell membranes 14 and 16, and a tail gas burner 12 all contained withina single housing 18. The housing is made of a thermally conductivematerial such that all of the components within the housing can operateat substantially the same temperature. Thus, the housing facilitates theformation of a zone that is substantially isothermal.

The housing 18 in FIG. 1 includes within it all of the flow routingmeans for distributing fuel and air to the fuel cell(s). The fuel stream20 passes out of the fuel reformer 10, along the anode side of the firstfuel cell 16. The fuel stream 20′ then passes along the anode side 22 ofthe second fuel cell 14 and finally into the tail gas burner 12. The airstream 26 passes (by means of internal routing channels not shown) alongthe cathode side 24 of the fuel cells 14, 16 and culminates into thetail gas burner 12 where the excess air is available for the combustionof unused exhausted fuel. (The air entrance to the tail gas burner doesnot appear in FIG. 1.)

Also shown in FIG. 1 is an insulating volume 28, which separates thehousing 18 from an outer wall 30 of the apparatus 5. The outer wall issubstantially maintained at a temperature that is at or near the ambienttemperature of the electrical device powered by the fuel cell apparatus.For efficient operation of a solid oxide fuel cell apparatus, thetemperature within the housing should be greater than 400° C., withbetter operating efficiencies obtained if the temperature is maintainedin excess of 550° C., 600° C., or 750° C. The ambient temperatures ofexternal electrical circuits and the outer wall 30 of a fuel cellapparatus will typically be in the range of 0° C. to about 60° C.Therefore, in this embodiment, a large thermal gradient in excess of300° C. is desirably maintained not only through the thickness of theintervening insulating volume 28, but also along fluidic connections 32,electrical connections 36, and along mechanical supports 38.

The insulating volume can incorporate insulation to substantially reduceheat dissipation from the housing. Thus, a partial vacuum can be formed,within the insulating volume or a low thermal conductance material canbe added to the insulating volume. An infrared radiation shield 40 canalso be disposed within or upon the fuel cell apparatus. It isbeneficial to maintain the required low level of total gas pressure inthe insulating volume when fabricating a low pressure or vacuuminsulation embodiment. For this purpose, it is useful to add a gettermaterial 42 which has the capability of absorbing background gases andmaintaining high levels of vacuum over the operating life of the device.A non-evaporable getter, which can be activated through electricalheating, is useful for this purpose, such as the SAES getters ST 171device (www.saesgetters.com).

The integrated fuel cell contained within a housing can have a totalthickness of 2.5 mm. In FIG. 1, two fuel cell layers 14 and 16, andthree routing layers 46, 48, and 50 are present, each with 0.5 mmthickness. Each of the two fuel cell layers is capable of producing 0.4W/cm² of electrical power. As a result, an exemplary integrated fuelcell apparatus is capable of delivering (2*0.4)/2.5=3.2 W/cc of powerdensity.

The housing, which integrates the functions of a fuel reformer, a set offuel cell membranes, a tail gas burner, and all internal fluid manifoldsin one thermal zone, can be fabricated through any number of fabricationtechniques. In particular, embodiments of the invention can befabricated using MEMS techniques (micro-electro-mechanical systems) ormicromachining techniques. Such techniques make it possible to integratethin film materials (for instance thin film electrolytes, anodes,cathodes and/or electrical connections) along with etched microchannelsfor control of fluid flow onto a common substrate that is thermallyconductive and mechanically robust. Structural support members areincluded in some embodiments as they are useful for patterning eitheranodes or cathodes into discrete regions. Individual membranes electrodeassemblies and fluid manifolds can be stacked together by a variety ofbonding techniques, to create fluid processing “systems.”

For example, an integrated housing can be assembled from a group ofsubstantially planar or non-planar semiconductor structures.Specifically, five silicon substrates can be bonded together to form the“box” that various fuel cell apparatus components are integrated within.Bonding together the five silicon substrates, results in a stackedconfiguration. In one embodiment, the substrates can be stacked asfollows: (1) fuel reformer substrate including fluidic interconnects;(2) a membrane electrode assembly, (3) a fluid routing layer, (4)another membrane electrode assembly, and (5) a top fluid routing layerincluding tail gas burner. Thus, a stack of layers can form some or allof the integrated fuel cell apparatus.

In a preferred embodiment, silicon is chosen as the substrate forbuilding the fuel cell membranes and other manifold structures. However,micromachining techniques also exist for building fluid flow channels inrigid wafers of glass and ceramic, all materials which possess the hightemperature strength required for solid oxide fuel cells. In order toprevent electrical shorting between different points of the membraneassembly, a silicon substrate can be coated with layers of silicon oxideor silicon nitride to render it electrically insulating.

Etched fluidic microchannels are formed in the above substrates by avariety of techniques, including wet and dry chemical etching, laserablation, diamond milling, tape casting, or injection molding. A varietyof substrate or wafer bonding techniques are available including fusionbonding, anodic bonding, sealing by means of eutectic solder materialsor thin films, or sealing by means of glass frits.

Fuel cell assemblies, including the anode, cathode, and electrolyte canbe deposited by a variety of thin and thick film deposition techniquesincluding sputtering, evaporation, chemical vapor deposition, laserablation, screen-printing, dip coating, or vapor spray techniques.

The preferred material for the electrolyte is yttria-stabilized zirconia(YSZ), although a variety of doped ceria materials are also availablefor this purpose. The preferred material for the anode of the fuel cellis a cermet of nickel and YSZ, although other catalytic metals may beemployed such as Pt, Pd, Fe or Co, and other oxide matrix materials canbe used such as ceria. The preferred material for the cathode of thefuel cell is lanthanum (strontium) manganate (LSM), although othercathode materials have been described including lanthanaum (strontium)cobaltite (LSC) and lanthanum (strontium) cobalt-ferrite (LSCF). Thepreferred material for thin film electrical connections in the fuel cellis platinum, although lanthanum chromite has also been described forthis application.

FIG. 2 is a further illustration of the fuel cell apparatus of FIG. 1,emphasizing the arrangement of fluidic connections and a heatrecuperator 34. The integrated fuel cell apparatus' housing 18 is shownonly in its external aspect, with sub-regions denoting the suggestedplacement of a fuel reformer 10, and a tail gas burner (or catalyticconverter) 12. A mixture of fuel and air enters along an inlet tube 60directly to the fuel reformer 10. After which, by means of internalrouting channels, the reformed fuel passes by the anode of the fuelcell, eventually ending up in the region of the tail gas burner 12. Airfor the cathode of the fuel cell enters through an inlet tube 62 andflows internally via a controlled route to the cathode of the fuel cell.Both air and fuel streams are finally re-united in the tail gas burner12 for extraction of any residual heat of oxidation before exiting thehot zone through an exit tube 64.

The inlet and outlet tubes bridge the region between the housing and thecold outer wall and should be designed for low thermal conductivity. Asan example, these tubes can be composed of silicon nitride, preferablywith wall thickness of 5 microns or less, such as are describedInternational Publication No. WO 03/013729. Alternatively, the tubes canbe made from silica glass capillaries. For example, glass capillariesare available with 1 mm outer diameters and wall thicknesses of only 125microns. The thermal power that will be conducted along such capillariesif they are 5 mm long and span a temperature gradient of 800° C. is only0.05 watts.

It will be recognized by those skilled in the art that otherarrangements of the fuel reformer and tail gas burner within the housingare within the scope of this invention. Similarly, other arrangementsand different numbers of inlet and exit tubes are possible than thoseillustrated in FIG. 2. For instance, for larger fuel cell apparatus itmay be preferable to add a fourth tube for delivering independent flowsof fuel and air from an external flow regulation system directly intothe fuel reformer. It may also be preferable to provide two independentsources of air into the cathode region, such that fluid pressure dropsare more effectively managed within the fuel cell apparatus and/or as ameans for controlling fuel cell voltages in local regions of the fuelcell membranes. Also, concentric tubes can also be used in certainembodiment.

Heat Recuperator

Again referring to FIG. 2, the heat recuperator 34, shown as two bars,is a means for heat recuperation and can be built as an integral part ofthe fluid tube assembly. The heat recuperator is typically made of athermally conductive material, such as silicon, such that the heat ofthe exhaust gases passing through the exit tube 64 can be absorbed andtransferred to the incoming gas streams in the inlet tubes 60 and 62.

As shown in FIG. 1, improved performance is possible by placing the heatrecuperator 34 within the insulating volume 28. In this position, thevarious internal temperatures of the heat recuperator can be maintainedintermediate between the temperature of the integrated fuel cellapparatus and the outer wall. Placing the heat recuperator within theexisting insulating volume also reduces the overall system size byeliminating separate insulation around the heat recuperator. Further,aligning the thermal gradient of the heat recuperator with the exitingthermal gradient between the integrated fuel cell apparatus and theouter wall decreases the heat loss from the heat recuperator becausethere is little if any temperature difference between a given section ofthe heat recuperator and the adjacent insulating volume.

Various means of heat recuperation are possible, other than the paralleltube arrangement shown in FIG. 2. For instance, a tube-in-tubecounterflow arrangement is appropriate or a stack of thin metal sheetsformed to allow for a counterflow by means of machined or shapedmicrochannels. Many other arrangements fall within the scope of thisinvention, as long as the physical placement of the heat recuperator iswithin the intermediate region between the isothermal (“hot”) zone ofthe fuel cell apparatus and the cold outer wall.

Low Thermal Conductance Fluid Connection

A general goal of the invention is to manage the total heat dissipationaway from the housing. In one particular element, to manage the heatloss through the tubes, (Q_(tubes)), which accounts for the solidconduction of heat along the length of fluidic inlet and exit tubes, theheat loss through the tubes can be calculated from the product of a) thethermal conductivity of the tube wall material, b) the temperature dropalong the tube, and c) the cross sectional area of the tube wallmaterial, divided by d) the length of the tube.

For small fuel cell apparatus systems, a maximum heat loss allowedthrough the fluidic tubes is determined to improve system efficiency.That heat loss, Q_(tubes), is desirably maintained below 0.1 watts pertube, preferably less than 0.05 watts per tube. This heat loss value issignificantly below the embodiments known in the art, however, systemefficiency improves dramatically when the fluidic connection tubes areconstructed with heat loss below this critical value. Table 2 showsexamples of typical known tube materials and design and exemplary tubes(embodiments 3 and 4) suitable for use with the present embodiments thatare constructed to satisfy the critical heat loss condition. TABLE 2Comparison of fluid connection tube materials. (The power loss Q assumesa total temperature drop of 700° C.) Thermal wall tube length/ powerloss conductivity thickness diameter per tube: Q Tube material W/cm-k)(microns) (mm/mm) (watts) embodiment 1 ⅛″ 0.25 325 30/3 1.9 stainlesssteel tube embodiment 2 stainless 0.25 125 20/1 0.35 steel capillaryembodiment 3 thin wall 0.4 2   3/0.5 0.03 silicon nitride embodiment 4glass 0.01 125 5/1 0.05 capillary

In a 33% efficient, 2 watt fuel cell apparatus generator, the fuel cellapparatus would be expected to burn an equivalent of 6 watts of fuel anda thermal loss of 0.1 watts per tube would represent only 5% of thetotal power consumed. For larger fuel cell apparatus in the range of 5to 30 watts either more tubes or tubes with larger cross section may benecessary to handle increased amounts of fluid flow. By maintaining thethermal loss of each tube below 0.5 watts, and preferably below 0.1watts, the percentage of thermal loss due to fluid connections can bemaintained at or below 10%, and preferably below 5%, of the total powerburned as fuel in the device.

Low Thermal Conductance Electrical Connection

Another general goal of the invention is to reduce the heat lossrepresented by solid conduction along electrical connections. In apreferred embodiment, the value of heat loss per electrical wire shouldbe less than 0.5 watts, and more preferably less than about 0.1 watts.An electrical loss of 0.1 watts or less per wire, however, requires theuse of higher resistance and finer diameter wire connections. Table 3shows the correlation between wire diameter, wire resistance, and heatloss for known wires and those useful in the invention (embodiments 3and 4). Note the inverse correlation between wire resistance and thermalpower loss along the wire, which is typical for metal conductors. Forknown fuel cell systems, where stack powers are typically in excess of100 watts and the total heat dissipated is greater than 300 watts, aloss of 1 watt per wire is not excessive. For fuel cell apparatus ratedat 20 watts or less, it is desirable to reduce the heat loss due to thewires. The method employed in this invention for controlling heat lossis to choose electrical connections where the electrical resistance isin excess of 0.1 ohms and preferably greater than 0.5 ohms. TABLE 3Comparison of electrical connection wires. (The temperature drop alongthe length of wire is assumed to be 700° C.) power loss wire wire wireper diameter length resistance wire wire material (microns) (mm) (ohms)(watts) embodiment 1 Cr/Ni alloy 800 30 0.1 0.34 embodiment 2 Pt 800 300.02 1 embodiment 3 Cr/Ni alloy 100 5 1.27 0.03 embodiment 4 Pt 50 50.81 0.02

From Table 3 choosing connecting wires for bridging the insulation spacewhere the resistance of the wires exceeds 0.5 ohms is advantageous. Toachieve an efficient fuel cell apparatus with this constraint, however,requires other changes to the fuel cell apparatus operating parametersand to the construction of the fuel cell stack. For instance, outputcurrents must be maintained at a levels low enough to prevent excessiveelectrical power loss by means of resistance in the connecting wires.Thus, using the techniques disclosed herein, currents can be reduced atany given power level by increasing the fuel cell voltage. However, inthe past, this objective was achieved by connecting or stackingindividual fuel cells in series such that voltages are added. For thisinvention, which deploys connector wires in excess of 0.5 ohms, astacked output voltage in excess of 10 volts is required, preferably inexcess of 15 volts.

One method for voltage stacking is an in-plane stacking, arrangement, inwhich fuel cell membranes layers are stacked vertically such that theanode of one cell makes electrical contact with the cathode of the celldirectly above it. A 10 volt output requirement for the fuel cell stackwould require that twelve to twenty fuel cell membrane layers beassembled in the vertical stack The embodiment illustrated in FIG. 1,however, depicts only two membrane layers due to volume efficiency.Nevertheless, an advantageous output voltage is possible using thein-plane stacking concept disclosed herein.

FIG. 3 illustrates the concept of in-plane stacking. In-plane stackingrequires the ability to pattern anodes, cathodes, and electrolytes suchthat series type voltage connections can be made. In FIG. 3, in anode 22of fuel cell electrolyte 23A is allowed electrically to contact acathode 24 that is disposed behind an adjacent fuel cell electrolyte23B. An interconnect material 25 allows for a low resistance electricalconnection between anode 22 and cathode 24. Structural support membersshown in FIG. 1 are also useful for patterning either anodes or 20cathodes into discrete regions.

Given the compact nature of the integrated fuel cell apparatus shown inFIG. 1, and the goal that electrical connections be achieved with narrowgauge wires (diameters less than about 100 microns), it is alsodesirable to provide a reliable method for attaching the connector wireswithout the use of bulky screws or crimp connectors. In one embodiment,the narrow gauge wires should be attached to both the integrated fuelcell apparatus and the connector strip at the outer wall by means of ahigh temperature brazing alloy or preferably by bonding methods such asa thermo-mechanical bond.

Isothermal Nature of Integrated Fuel Cell Apparatus

The efficiency of a solid-oxide fuel cell apparatus improves when allthe functions of fuel reformer, fuel cell and tail gas burner integrateinto a single housing with minimum surface area. Efficiency alsoimproves when the housing is designed with sufficient thermalconductivity to enable an efficient distribution of heat or sharing ofthermal energy between components. In particular, the tail gas burnercan be used to share supplemental heat that improves overall efficiency.Thus, the thermal energy generated in the tail gas burner maintains ahigher and more efficient operating temperature in the fuel cellapparatus. In this fashion, the thermal stresses and costs associatedwith heat up or cool down of the device are reduced.

Furthermore, improved fuel cell efficiency is possible by operating thefuel cell at higher voltages, closer to an equilibrium electrochemicalpotential. Such an operating condition implies the generation of lesswaste heat when compared to operating at a lower fuel cell voltage. Therequired amount of thermal energy for maintaining operating temperatureis attainable by extracting heat from the combustion of under-utilizedfuels in the tail gas burner.

Several methods can be employed to maintain sufficient thermalconductivity and nearly isothermal operation between components withinthe integrated fuel cell apparatus. Silicon, used as a substratematerial is an excellent thermal conductor at elevated temperatures.Glass or ceramic substrates are suitable material choices based onthermal conductivity, as long as their resultant wall thicknesses aresubstantially in excess of 100 microns and preferably in excess of 300microns. The thermal conductivity of glass substrates is enhanced by thedeposition of metallic thin films over areas that are not electricallyactive, such as the outer surfaces of the housing. Candidate thermallyconductive metal coatings include chromium, gold, and platinum.

As a means of enabling the substantially isothermal operation of thesystem, it is helpful to design the integrated housing such thatseparate components (fuel reformer, tail gas burner and the fuel cellmembranes) share between any pair of them at least one common structuralwall. This wall could be an outer wall of the housing or it could be aninternal wall formed, for instance through the bonding of individualsubstrates.

By sharing structural walls and by providing substrates with sufficientthermal conductivity, it is possible to maintain any temperaturedifferences between components during operation to less than 150° C.,preferably less than 50° C.

Power Density

When designing a portable solid-oxide fuel cell apparatus, it isimportant to determine a minimum thickness of insulation material thatis adequate for maintaining high operating temperatures withoutexcessive consumption of fuel energy. The amount of heat that willdissipate from an integrated fuel cell apparatus is proportional to itssurface area. An integrated fuel cell apparatus designed for a 5 wattapplication, therefore, becomes difficult to insulate efficiently sinceits surface-to-volume ratio is much higher than an integrated fuel cellapparatus designed for applications at 20 watts or more.

The power density of the integrated fuel cell apparatus is a significantdesign parameter. In particular, the power density may be the designparameter that most influences the final efficiency and size of theinsulated package. The power density of the integrated fuel cellapparatus, expressed in watts per cubic centimeter (W/cc), determineshow much surface area is exposed for every watt of electricity produced.As a result, the influence of integrated fuel cell apparatus electricalpower density on final package size is large and disproportionate. Forexample, an integrated fuel cell apparatus which is capable of producingpower at 5 watts and 1 w/cc will require a package size, includinginsulation, of 66 cc. In contrast, an integrated fuel cell apparatusrated at 5 watts and 2 w/cc can be insulated inside of a package of only17.8 cc. Therefore, a two-fold increase in power density results in a3.7 times decrease in package size with no loss in thermal efficiency.(This example assumes the use of an aerogel™ insulation rated at 0.04W/m-K, maintaining a temperature drop of 800° C.)

FIG. 4 shows another embodiment of the present invention, in this case alarger fuel cell apparatus 105 employing four different membrane layers.Each layer, whether a fuel cell membrane 114, an air or oxygen routinglayer 148, or fuel routing layer 147, 149, 150, is about 0.5 mm or lessof thickness, such that the total stack is about 4.8 mm in height. FIG.4 also includes within its housing a fuel reformer 110 and a tail gasburner 112 constructed as part of layer 146. The fuel routing layerscarry fuel out of the fuel reformer past their respective fuel cellmembranes and/or carry exhaust into the tail gas burner after passingtheir respective fuel cell membranes. Using FIG. 4, the average spacingbetween membrane layers, defined as the total integrated fuel cellapparatus height (4.8 mm) divided by the number of membrane layers (4)can be calculated. The average membrane spacing of FIG. 4 is thereforeabout 1.2 mm. In this case the power density can be derived by dividingthe average power density of each fuel cell layer (0.4 W/cm²) by theaverage membrane spacing, resulting in a power density of about 3.3W/cc.

Construction of the fuel cells stack to enable greater than about 2watts of electrical energy per cubic centimeter of integrated fuel cellapparatus volume is preferable. It is also desirable to operate a givenfuel cell stack in such a way to produce greater than 2 W/cc. The powerproduced by a fuel cell can be controlled by varying the voltage, aswell as by varying the temperature of the fuel cell. Larger fuel cellsare typically operated at voltages above maximum power in order toincrease the efficiency of the chemical to electrical energy conversion.Power densities greater than 1 W/cc, 1.5 W/cc, or preferably 2 W/cc, areincluded in the present invention.

Increasing the voltage to a level which lowers the power out below about2 W/cc actually lowers overall system efficiency in small systemsbecause insufficient heat is produced to maintain the requiredtemperatures. The integration of a catalytic converter or tail-gasburner allows for some decrease in fuel cell power output.

One significant power density improvement is achieved through closervertical spacing between membranes. The average spacing betweenmembranes in the existing art is in the range of 2.5 to 4 mm, while theaverage spacing in the invention typically is less than about 1.5 mm,approaching values as small as 1.0 mm. The advantage of closer membranespacing is derived from two advantageous structural features: a) the useof mechanically robust composite membrane designs, and b) the use ofstructurally simple flow routing layers that are enabled by the use ofin-plane stacking. In this embodiment, advantageous use is also made ofthe architecture of in-plane fuel cell stacking. In-plane fuel cellstacking makes possible a number of structural advantages that togetheract to reduce the spacing between membranes and increase the powerdensity to values well in excess of 2 W/cc.

The use of composite membrane structures has been described in co-ownedInternational Publication No. WO 2005/030376. Briefly, compositemembrane structures make possible the combination of a strong structuralsupport member in combination with thin (<2 μm) YSZ membrane layers.Such a structure has the strength to withstand the stresses of thermalcycling without the need for excess substrate thickness and can beachieved using silicon wafer thicknesses of about 0.5 mm or thinner.Similar composite structures can be built from dense ceramic substrates,for instance Al₂0₃ materials, regardless of coefficient of thermalexpansion, to the extent that they obey the design rules laid-out in theabove-identified patent application.

In known layer fabrication techniques, a gas-impermeable bipolar plateis required to separate gas flows between fuel and air. A verticalplanar stack requires that electrical contact be made from the anode ofone membrane layer to the cathode of the adjacent layer. However, thefuel that passes over the anode must not be allowed to mingle with theair that flows over the cathode. Therefore an electrically conductivebipolar plate is typically employed which effects not only theelectrical connection between layers but also the routing of fuel to theanode, air to the cathode, and a hermetic separation between the gasflows.

Returning to FIG. 1, no such gas separation is required in the flowrouting layers as the cathode of fuel cell membrane 14 directly facesthe cathode of fuel cell membrane 16. Both membrane layers share thesame gas flow and no electrical connection is required between these twofuel cell layers. Therefore, the design of the flow routing layer issimplified and extremely thin flow routing layers are possible, withthicknesses in the range of 0.3 to 0.5 mm.

FIG. 5 illustrates one such flow routing layer, having geometrycompatible with the four layer fuel cell stack shown in FIG. 4. Openings180 provide for vertical passage of the fuel from one layer of the stackto layers above or below. Channels 182 provide for the flow of air overthe cathode. To the extent that flow routing layer 148 separates twocathode-facing layers, only a simple ribbed structure is necessary toadd both structural rigidity to the stack and provide for sufficientdistribution of air over all cathode surfaces.

The flow routing layer can be composed of a rigid material such assilicon. Choice of silicon in this embodiment has the further advantageof matching the structural materials between all of the membrane layersand the flow routing layers. In this fashion, one can avoid the stressesassociated with differing thermal expansion coefficients between thesetwo structural materials.

The flow routing layer can be machined or stamped from a metallicmaterial. However, the coefficient of thermal expansion of the flowrouting layer must remain substantially similar to that of thestructural material in the membrane layer. Thin metallic flow routinglayers will not be as rigid as a routing layer built from silicon, butthe silicon or other ceramic material employed for the membrane layerwill provide more than enough rigidity and provide sufficient strengthto the overall stack to withstand the stresses of thermal cycling.

Heat Generation/Insulation

In addition, to maintaining electrical power output above about 2 W/cc,system performance and size are also improved if the thermal heatgenerated is maintained above 2 W/cc. Due to the rapidly increasingsurface area at the small sizes, it is desirable to maintain asufficiently high heat density in order to maintain the operatingtemperature of the device. If the fuel cell apparatus alone does notproduce enough heat, using a tail gas burner to combust extra fuel inorder to maintain greater than 2 W heat per cubic centimeter isadvantageous for efficient device operation. Ensuring that the devicewill be operated at greater than 2 W heat per cubic centimeter allowsthe insulation thickness to be minimized, thereby producing a devicewhich is commercially competitive with existing batteries.

Design of the insulation volume in the solid-oxide fuel cell system isanother area for improving solid-oxide fuel cell efficiency. Fibrous ormicro-porous ceramics have been utilized for the function of isolatingthe high temperature housing from the outer package and its environswhile minimizing the amount of waste heat that is lost by conductionthrough the insulation. Aerogel materials are available, for instance,which possess low thermal conductivities and are stable for operation at800° C. as low as 0.04 W/m-K.

Perhaps the most space-efficient insulation, particularly for smallpackages, is a vacuum insulation. This allows portions of the fuel cellapparatus to function as a thermos bottle with the outer walls andinsulating volume maintaining the contents integrated within the housingat a desired temperature. By maintaining total gas pressures in theinsulating volume of less than 100 mtorr, preferably less than 20 mtorr,more preferably less than 10 mtorr, it is possible to substantiallyeliminate any thermal loss by means of conduction away from the housingthrough the gas phase. A partial vacuum may be formed within theinsulating volume bounded by the outer wall by evacuation with a vacuumpump, through an outgassing port, or alternatively, by performing theprocess of sealing-together the elements of the outer wall within anevacuated 20 atmosphere.

When utilizing the embodiment of a vacuum package, and eliminating useof a thicker solid insulation material such as aerogel, a new type ofthermal loss from the housing becomes an issue in the form of thermalloss by means of infrared radiation. Infrared radiation emanating fromthe surfaces of the housing can become, in fact, the dominant heat lossmechanism for the insulation package illustrated in FIG. 1.

There are at least three methods for reducing the thermal loss byradiation, any one of which may be used singly or in combination. Thesecan be seen by returning to FIG. 1. First, a reflective coating isapplied to the outer surfaces of the integrated fuel cell apparatus,reducing thereby the infrared emissivity and power loss from the hotsurface. Second, a radiation reflector 40 can be provided along theinner surfaces of the vacuum outer wall 30 for the purposes of returninginfrared radiation back to the integrated fuel cell apparatus. Thisradiation reflector can be constructed by means of a metallic coatingwhich is deposited on the inner surfaces of the outer wall 30, or bymeans of a metallic or infrared reflective material which ismechanically attached to the inner surfaces of the vacuum wall. Inaddition, a series of parallel infrared reflectors can be providedbetween the hot surface of and the cold surface of the outer wall.

Fuel Cell Apparatus Regulation, Monitoring, and Safety

As discussed above, integrating a fuel reformer, a fuel cell and a tailgas burner within a substantially isothermal zone improves theefficiency of the fuel cell apparatus and makes it a suitable batteryreplacement device. Although the increased energy density of a batteryreplacement improves the commercial value, localizing a large amount ofchemical and thermal energy in a small volume increases the likelihoodof uncontrolled combustion, explosion, and/or the release of harmfulchemicals. Because many of the devices disclosed herein are suitable foruse by consumers, e.g., a cellular telephone battery replacement, it isdesirable to incorporate safety and device monitoring features in thefuel cell apparatus.

Although not always expressly emphasized in the description of theembodiments provided above, many of the structural and chemical flowaspects of the invention as previously disclosed herein inherentlyenable the safe operation of the fuel cell apparatus. For example, whilethe insulating volume discussed above increases the energy density inthe isothermal zone, by containing the heat, the insulating volume alsoshields the users of the device from excess heat. Simultaneously, theinsulating volume can serve as a capture zone to stop any uncontrolledcombustion and act as a means for terminating fuel conversion reactionsshould the insulating volume be breached or otherwise penetrated. Theseand other safety features are discussed in more detail below.

There are multiple design strategies for improving the safety of a fuelcell apparatus, in general, and portable solid-oxide fuel cell devices,in particular. However, for organizational purposes, the safety featurescan be grouped into two broad categories, noting that variousembodiments may include features that place them in both categories.First, there are passive design structures and methods, such as the heatcontaining benefits of the insulating volume. Some passive safetyfeatures operate persistently in the background and respond to apparatuschanges without the need for directed activation. For example, thedesign aspects of fuel cell apparatus geometry to control combustionpropagation represents another passive safety feature.

The second category of safety features includes active methods anddevices. An exemplary active safety feature is an electronic controlsystem that terminates device operation in response to some event, suchas sensor alarming when a conduit is blocked, and appropriatelyinitiating a combustion termination instruction in response to thealarm. Generally, control system/sensor systems fall into the activesafety feature category.

Again, although the terms passive and active are used, it is understoodthat they are used for organization purposes and are not intended tolimit the scope of the description or claims. Thus, although describedas active or passive, various safety features may include both activeand passive elements without limitation.

Prior to discussing each safety feature in greater detail, it will beuseful to introduce the different active and passive device and methodembodiments that may be integrated with the fuel cell apparatusdisclosed herein. As combinations of these different features, as wellas the other embodiments discussed herein are possible, a broad range offuel cell apparatus can be fabricated by those skilled in the art.

Some of the passive safety features include: insulating volumes andother forms of device insulation; regulating conduit diameters torestrict the emission of volatile compounds; regulating thermal energylevels in the device through the choice of structural materials;arranging device components, such as the fuel reformer and tail gasburner, to enable self-regulating temperature levels that control fuelconversion reactions; regulating various device geometries such as thediameters of various flow streams and conduits to control combustionpropagation; and incorporating device components, such as a tail gasburner, to provide substantially non-volatile exhaust by pre-reactingvolatile fuels and/or fuel conversion byproducts.

In turn, some of the active safety features include: integratingmechanical and/or electronic sensors to monitor the operation of thefuel cell apparatus and its sub-systems; using control elements, such asshut off valves, to terminate device operation in response to an alarm;control systems that incorporate sensors and control elements to preventunsafe device operation; authentication devices to verify thesuitability of a given fuel source for use with the fuel cell apparatus;and various circuits to provide for identification, authentication,control, monitoring, and operation of the fuel apparatus and itscomponents.

Therefore, the invention further includes features that provide “failsafe” operation such that any indicia of device failure or atypicaloperation triggers a shutdown of some or all components of the device.Thus, effects of a potentially disastrous failure are reduced byterminating device operation in advance of an energy release.Conversely, passive techniques that operate in the background duringoperation to increase safety, such as insulating foam that protects theuser from heat, are also within the scope of the invention.

FIG. 6 illustrates a schematic diagram of portions of an embodiment of afuel cell apparatus 200 incorporating various active and passive safetyfeatures. A solid oxide fuel cell 202, a fuel reformer 204, and a tailgas burner 206 are in fluid and thermal communication with each other. Aportion of the collection circuit 207 used to collect the electricityproduced from the fuel cell 202 is also shown. Additionally, all threecomponents are integrated together within a housing 208 in an isothermalzone that is bounded, in part, by an insulating volume 210. In turn, theinsulating volume 210 is defined, in part, by an outer wall 212 of thefuel cell apparatus 200 and portions of the fluidic manifold and devicepackaging 214. The volume of the housing can range from about 0.5 cc toabout 100 cc. In one embodiment, the volume of the housing ranges fromabout 0.5 cc to about 10 cc. In another embodiment, the volume of thehousing ranges from about 1 cc to about 5 cc. Additionally, theinsulating volume is less than or equal to about 200 cc.

Fuel 215 is typically contained within a fuel tank or fuel cartridge216. The fuel tank 216 can further include a fuel tank electricalconnection 218 and a fuel tank fluidic connection 220, the fuel tank 216also can include an authentication circuit 222 in communication with thefuel tank electrical connection 218 or as an independent, addressabledevice. An air pump 223 can also be integrated with the fuel cellapparatus 200 to provide air necessary to sustain combustion with theisothermal zone. One or more device control systems 224 can be includedto receive and operate in response to sensor data and/or circuitinputs/outputs.

Various conduits and conducting elements can be used to facilitate theflow of fuel, air, combustion by-products, and other compounds to andfrom the fuel cell apparatus. As various conduits and transportmechanisms can be used, it is useful to illustrate the flow streams orpaths rather than specific structures. However, suitably sized conduits,channels, or other fluid transport structures can be used to contain theflow streams as known to those of skill in the art.

Thus, various flow paths/streams F₁ through F₅ are depicted in FIG. 6.Flow path F₁ illustrates the flow of fuel 215 into the device while flowpath F₂ illustrates the introduction and flow of air into the device. Inturn, flow path F₃ illustrates the mix of air and processed fuel leavingthe fuel reformer 204. After the initial reaction in the fuel cell 202,chemical compounds and reaction by-products flow to the tail gas burner206 along flow path F₄. Finally, after processing by the tail gas burner206, thermal energy and substantially non-volatile exhaust flows fromthe apparatus 200 along flow path F₅.

In part, the invention provides techniques for regulating the operationof a solid oxide fuel cell by controlling where different reactionsoccur within the apparatus. In one embodiment, fuel is converted tohydrogen and by-products at a reaction temperature T₁ within the fuelreformer. The reaction temperature is within a reaction temperaturerange. For example, as discussed below in FIG. 8, the reactiontemperature range for butane is from about 200° C. to about 800° C. Theby-products are converted to exhaust products and thermal energy. If theexliaust products are non-volatile such as water, oxygen, and/or carbondioxide, they flow from the device along flow path F₅.

If volatile compounds are still detected along flow path F₄, the tailgas burner can further clean, for example by substantially completeoxidation, the exhaust flow before it enters the environment. Inaddition, the thermal energy from the step of converting the by-productsassists in maintaining the reaction temperature range. As a result, thedifferent device components within the isothermal zone can be used topre-heat incoming fuel and maintain the temperature of the fuelreformer. Consequently, each component can act as a control element toterminate or initiate fuel conversion.

Returning to FIG. 6, a control system/control circuit 224 can be used tointerface with the authentication circuit 222 of the fuel tank 216. Inone embodiment, the control circuit 224 also receives data relating tothe operation of the fuel cell apparatus 200 and regulates the operationof the fuel cell apparatus in response to that data. A temperaturesensor 226, a flow sensor (or sensors) 228, pressure sensors (notshown), and other sensors and detectors can be incorporated within andexterior to the various components of the fuel cell apparatus to monitorvarious operational parameters. An electrically/mechanically controlledflow valve 230 may be associated with the fuel conduits or may be partof the fuel cartridge. Suitable sensors can include, but are not limitedto, a fluid flow detector, a chemical detector, a pressure detector, acomparator circuit, a voltage detector, a current detector, a directmass flow rate detector, an indirect mass flow rate detector, a volumeflow detector, a differential detector, a temperature detector, aradiation detector, and combinations thereof.

The control system can send and receive apparatus data via variouselectrical connections not explicitly depicted in the diagram. Thus, theapparatus may start to operate irregularly and pose a risk to a user.The control system may identify the irregularity via a temperature data,flow data, pressure data, preset data, or other data. In response, thecontrol circuit can send an electric signal to a flow valve or anotherfuel cell device component, such as the fuel reformer, to stop fuelconversion. Thus, for a valve-based system, the control system instructsthe valve to close and terminate the flow of fuel to the fuel reformer.Suitable apparatus data can include, but is not limited to, temperaturedata, fluid flow data, pressure data, radiation data, electric signaldata, electric current data, voltage data, geometric data, structuralstability data, vibration data, sheer stress data, chemical compositiondata and combinations thereof.

The sensors, conduits, valves, controls, and circuits may be located invarious positions in the system as would be obvious to one skilled inthe art. For example, the fuel flow sensor may be located upstream of anelectrically controlled valve rather than downstream as shown in FIG. 6.Additionally, although the various electrical connections are not shownbetween the electronic control circuit and the various sensors, valvesand pumps, the appropriate connections can be achieved through variousconduits, semiconductor tracings, microfluidic channels and wire basedconnections as known to those of ordinary skill in the art.

Various sensors known to those of ordinary skill in the art areincorporated in the apparatus in different embodiments. In particular,suitable sensors include, but are not limited to thermocouples,unsheathed fine wire thermocouples, Type R, 0.001″ diameter, such asmodel number P13R-001, made by Omega Engineering, Inc.; platinumresistive temperature detector (RTD) such as model number WS81 (OmegaEngineering, Inc., One Omega Drive, Stamford, Conn. 06907-0047, USA);and flow rate sensors, MEMS flow sensors such as model D6F (OmronElectronic Components, 55 Commerce Drive, Schaumburg, Ill. 60173 USA).

Sensors can be positioned to directly detect a particular parameter ofinterest or indirectly positioned to capture data from differentsources. A thermal sensor can be positioned to capture indirect heatthat propagates along a flow path, even though it is integrated in thedevice to measure the temperature of the originating heat source.

Various sensors and control elements useful in the invention include,but are not limited to, a fluid flow detector, a pressure detector, acomparator circuit, a voltage detector, a current detector, a directmass flow rate detector, an indirect mass flow rate detector, a volumeflow detector, a differential detector, a feedback loop, a temperaturedetector, a radiation detector, a valve, a unidirectional flow device, agasket, a seal, a gate, a membrane, an iris, an occluder, a vent, aconduit, and combinations thereof.

The control system(s) represent an active design solution for ensuringsafe operation of the fuel cell apparatus. FIGS. 7A and 7B illustratecontrol flow diagrams that regulate device operation in response to flowdata and temperature, respectively. The control flow in FIG. 7A relatesto a device where fluid flow sensors are added to all input and outputfluid streams which are connected to the isothermal zone. Anelectronically controllable valve is also included that controls theflow of fuel into the fuel reformer. An electronic circuit such as acomparator circuit or other suitable circuit that compares the totalincoming flow to the total outgoing flow is incorporated within theoverall control system. As shown in FIG. 7A, if the flows do not match,then a component in the fuel cell apparatus may be malfunctioning. Thecontrol system automatically addresses electronically the fuel valve,causing it to close and restrict or cease the flow of fuel. The blockageof the flow path prevents any continued, unsafe release of fuel andisolates the fuel source from the isothermal zone. The blockage alsoprevents further fuel conversion and the creation of additional thermalenergy.

In one embodiment, the mass flow rate of the output and input streamsare substantially equal in normal operation. However, the volume flowrates may be different even in normal operation, e.g., due to chemicalreactions or temperature changes. The flow sensors can be direct orindirect mass flow sensors, although other sensors can be used asappropriate. In other embodiments, the flow sensors are volume flowsensors, and the electronic circuit corrects for the expected differencein volume flow rate. Of course, both mass and flow sensors can be usedtogether.

In FIG. 7B, another control flow suitable for use with a control systemto regulate the fuel cell apparatus is illustrated. The control flowdescribed in FIG. 7B is suitable for use with the device embodimentdepicted in FIG. 6 to the extent that the device includes a controlsystem, a controllable valve, and a temperature sensor. The temperaturesensor is used to measure the temperature in the isothermal zone thatincludes at least one of a fuel cell, a fuel reformer, and a tail gasburner. A preset temperature or temperature range can be determined forthe device during operation. Additionally, temperature presets can bedetermined for various individual device components such as the fuelreformer, the fuel cell, and the tail gas burner. The temperature presetcan be calculated based on where the sensor is positioned, the levels ofintervening insulation, the type of fuel being used, and other relevantfactors.

As shown in the control flow in FIG. 7B, the actual temperature in theisolated thermal region or other area of interest within the apparatusand the predetermined temperature preset are compared. If the valuesmatch, the device is deemed in a normal operation state and themeasurement is repeated. In one embodiment, a range of acceptable valuesare allowed such that an exact match with the preset value is notrequired. However, if the temperature measurement and the presettemperature are substantially different, the control system restrictsthe flow of fuel or otherwise terminates device operation. Other activecontrol flow approaches that make use of a sensor, a control system, anda fuel regulating element are also within the scope of the invention.

In addition to the control systems and sensor-based approaches describedabove, additional safety features can be incorporated in the fuel tankinterface portion of the apparatus. As discussed above with respect todevice embodiment of FIG. 6, a disposable or refillable fuel source,e.g., a fuel tank or a cartridge, can be fabricated to includeauthentication circuitry or mechanical interface portions to regulatedelivery of fuel. Thus, if a particular fuel cell apparatus is onlydesigned to work with butane from a certain vendor, the interface of thefuel cell apparatus may be designed with a particular geometry that willnot allow a standard butane fuel tank or a fuel tank containing anothertype of fuel to interface with the fuel cell apparatus. Fuel tank/fuelcell apparatus interface controls can be implemented using a mechanicallock and key model wherein certain interface portions on the tank andthe apparatus must fit together to enable fuel delivery. Patterns ofraised and lowered interface pins and grooves can also be used toaccomplish these access controlled fuel delivery objectives.

Alternatively, electrical contacts can be integrated into the fuel tankwith associated circuitry that connects to corresponding contacts andcircuitry in the fuel interface portion of the fuel cell apparatus. Thecircuitry portions on both the tank and/or the fuel cell apparatus cancommunicate with each to authenticate the fuel tank source and determineif fuel delivery should be allowed from the tank. If the fuel tank isnot properly authorized, then the fuel apparatus can electronicallyblock fuel delivery by engaging or failing to release a flow valve andallow fuel to pass into the apparatus. The fuel cell apparatus canelectrically query the fuel cell cartridge and interpret the passive oractive response signal. Alternatively, functional portions of the fuelcell control circuitry may be located in the fuel cell cartridge, forexample firmware or software. As a result, a user can ensure that thefuel cell cartridge is an authorized safe cartridge.

Integrating circuitry, either within the control system or within theinterface portion of the fuel cell, also allows the fuel cell apparatusto measure the contents of the fuel tanks and report how much fuel isremaining. Thus, inclusion of suitable circuitry allows the fuel cellapparatus to report via a graphic display or other alarm element howmuch fuel remains or that fuel will be fully consumed within a specifiedperiod, given the current usage level. The control system and/or variouselectrical components can monitor fuel consumption through use of a massflow sensor. The flow rate can then integrated over time, or sampled atfixed time intervals and stored, in order to determine the amount offuel consumed. This information can be periodically written to memory inthe fuel cell cartridge to maintain an accurate account of the fuelremaining in the cartridge.

The fuel tank or fuel cell cartridge can also include an electronicallywritable or reconfigurable counter device. This counter device ismodified by the fuel cell system as fuel is used by the system. As such,the amount of fuel drawn from the cartridge and/or the amount of fuelremaining in the cartridge can be tracked. This information helpssafeguard the user by ensuring that the fuel cell does not attempt tooperate when insufficient fuel is available. Thus, in one embodiment afuel reserve level can be preset. Therefore, once a control element orcircuit reports that the fuel source is at the fuel reserve level, fuelcell apparatus operation can be reduced or terminated. The counterdevice can also be used for billing purposes regarding fuel consumption.This feature can be combined with those embodiments discussed above torelating to an authentication system that shuts down the apparatus if apotentially unsafe cartridge is connected. An exemplary control flowrelating to a fuel consumption record embodiment is shown in FIG. 7C.

Some of the control system sensor based approaches discussed above canalso be modified to ensure that non-toxic and/or non-volatile compoundsare vented from the fuel cell apparatus as exhaust. The objective ofproducing substantially non-volatile exhaust can be achieved using bothpassive and active approaches. In the active approach, the incoming air,incoming fuel, and exhaust flow rates are measured. These different flowrates are then compared electronically to determine if substantially allthe fuel being released from the fuel storage device is being processedand exhausted by the fuel cell device. If excess fuel is being released,a determination that unprocessed fuel is being vented as exhaust can bemade, and the device can be shut down or the amount of fuel deliveredfrom the tank can be adjusted as necessary.

It is also possible to passively process the fuel stream using theexisting fuel cell apparatus components to limit a user's exposure tothermal energy and undesirable chemicals in the device's exhaust. Ifcombustion products are allowed to exit the device through an exhauststream, a user of the device may be exposed to toxic or explosivecompounds. Therefore, it is desirable to ensure that fuel cell apparatusexhaust has been scrubbed via supplemental heating to reduce the levelsof dangerous compounds.

To prevent any of the input fuel, e.g., butane, from making it throughto the exhaust, the tail gas burner and/or fuel cell oxidize or combustall of the exhaust to produce primarily water and carbon dioxide. Suchscrubbing also prevents the exhausting of any intermediate by-productsfrom a fuel reformer, such as hydrogen, carbon monoxide, formaldehyde ormethanol. The oxidation process also produces heat, as discussed earlierin this specification. The oxidation can occur, for example, in aseparate tail gas burner, or as part of the operation of the fuel cell.Other heat producing reactions other than combustion or oxidation thatcan occur within the fuel cell apparatus are also within the scope ofthe invention. The excess heat produced by these device components canbe used to maintain the reaction temperatures as discussed herein andensure substantially all vented by-products are rendered non-volatile.

Another type of device failure can occur if the tail gas burnermalfunctions while the fuel reformer continues to operate, causing theexhaustion of various intermediate fuels, some of which may be toxic.The invention integrates the fuel cell with the fuel reformer and thetail gas burner such that the heat from the tail gas burner or the fuelcell apparatus is used to maintain the operation of the fuel reformer.Such operation is achieved by balancing the heat loss through thesurrounding insulation with the heat generated by at least one of thefuel cell, the tail gas burner and the fuel reformer. During normaloperation, sufficient heat is available to maintain the fuel reformerabove a minimum operating temperature. However, in the case of a tailgas burner failure, less heat is available. As a result, the fuelreformer temperature will drop below a maximum “off” temperature andfuel conversion will cease. (e.g., the temperature details in FIG. 8relating to butane).

As a result, this temperature drop can stop or substantially reduce theproduction of intermediate products. Therefore, the arrangement ofdevice components provides a self regulating temperature control systemsuch that if one or more components fail, insufficient heat ismaintained to drive the fuel conversion reaction. Therefore, a devicecomponent failure shuts down the device before any harm occurs to auser. This feature, in combination with the control systems describedabove with respect to FIG. 6, allows for the fabrication of safe batteryreplacements using solid oxide fuel technology.

FIG. 8 shows some of the temperature characteristics for convertingbutane into energy in a fuel cell apparatus. Specifically, FIG. 8illustrates a data graph of butane conversion versus temperature showingefficient operation above roughly 500° C., and nearly zero conversionbelow roughly 300° C. The specific temperatures are geometry dependentand chemistry dependent (e.g. methanol conversion would have a lowerpair of temperatures). However, the graph suggests that knowing whichtemperatures levels result in the cessation of energy conversion allowthe reaction temperature ranges to be used as self limiting reactioncontrol parameters. As a result, these temperature ranges can beselectively used to turn the apparatus on and off as part of a safetycontrol system or during normal operation.

A fuel reformer within a particular fuel cell apparatus can be tailoredto process a specific fuel or class of fuels. Thus a fuel reformer canbe adapted to process butane as an input product that it partialoxidizes into hydrogen and carbon monoxide. For this butane partialoxidation embodiment, significant conversion occurs at 500° C., and morepreferably 600° C., 700° C. or 800° C. In contrast at lower temperaturessuch as below 400° C., little to no conversion occurs for this butanespecific embodiment. Furthermore, for the butane adapted fuel reformer,operation at or below about 300° C., about 200° C. or about 100° C.results in further decreases in fuel conversion. An alternate embodimentis based upon the steam reforming of water and methanol into hydrogenand carbon dioxide. In such a methanol steam reforming embodiment,minimum “on” temperatures are typically 200° C., 250° C., 300° C., and350° C. while maximum “off” temperatures are typically 200° C., 150° C.,100° C. or 50° C. Thus, when a portion of the device fails or theapparatus is otherwise cooled to one of these ranges, fuel conversion isterminated. In addition, to the temperature controls described hereinother passive approaches are also possible.

A mechanical break in a conduit, a seal, or a wall portion of the fuelcell apparatus may expose the hot zone or hot reaction by-products tothe external user or environment. One embodiment of the presentinvention uses a reduced pressure in the insulating volume. In thisembodiment, a mechanical break necessarily causes an increase inpressure within the insulating volume as the partial vacuum dissipates.In turn, this pressure change in the insulating volume causes a dramaticincrease in heat lost from the housing which includes at least one of afuel reformer, a fuel cell and a tail gas burner. In one embodiment, thegeometry of the insulating volume is such that the thermal conduction atreduced pressure is sufficiently low such that the housing is maintainedabove a minimum operating temperature. As discussed above, this minimumtemperature is necessary to sustain the fuel conversion reaction.Therefore, when the insulating volume is at or near atmosphericpressure, and insulating benefits cease, more heat is conducted awayfrom the housing. This heat loss and the associated cooling of thehousing below a maximum non-operating or “off” temperature prevent thefuel cell from operating.

The “off” temperature may be selected to meet a variety of safety oroperational requirements, for example the temperature may besufficiently low such that it provides minimal risk of injury. This canbe achieved by setting temperatures levels wherein no explosion canoccur via ignition, or alternately the “off” temperature may be selectedsuch that no toxic intermediates can be formed. Therefore, theinsulating volume temperature controls and reaction temperaturerequirements eliminate the risk of heat from an operating devicereaching a user if a mechanical breach in the device packaging occurs.

An apparatus failure can also occur as a result of a large temperaturespike due to small changes in the fuel flow rate, or a decrease in fuelcell efficiency. In an embodiment with a solid insulator, thetemperature is approximately linearly proportional to the heat produced.For example, if a system is constructed to operate at 800° C. with 4watts of heat input, and the heat input grows to 5 watts, then thetemperature may rise linearly to 1000° C. This 200° C. increase intemperature can cause dangerous failure conditions, such as materialmelting. In the present invention, the use of a reduced pressureinsulating volume, (rather than solid insulation) and the optional useof low conductance tubes and electrical connection elements, results inthe primary heat loss mechanism being thermally induced radiation.

The magnitude of heat lost through thermal radiation is proportional tothe fourth power of absolute temperature. As a result of thissuper-linear dependence, a small increase in heat input only results ina small increase in temperature. For example, if thermal radiation isthe only heat loss mechanism in a device constructed to operate at 800°C. with 4 watts of heat input, and the heat input grows to 5 Watts, thenthe temperature only rises to 862° C. This reduction in excess heat is asignificant improvement with regard to both safety and efficiency. Assuch, for this additional reason, using a low pressure insulating volumeallows for safe device embodiments that limit the risk of user's beingexposed to thermal energy. In one exemplary embodiment, the inventionrelates to a self-limiting fuel cell device operating at a nominaltemperature between 700 and 900° C. and packaged in a vacuum of lessthan 250 mT such that infrared emission is a significant heat lossmechanism.

When an apparatus enters a failure state or otherwise evidencesirregular behavior, it is desirable to reduce the likelihood ofcombustion propagating from within the hot region through to the outerenvironment. Combustion propagation can be regulated by sizing thediameter of the fluid conduction elements that define the various flowstreams in the device. Thus, the diameter of the fluid conductionelements, such as a flame arrester conduit, may be restricted to lessthan about 154% of the Maximum Experimental Safe Gap (MESG). Forexample, the National Fire Protection Association (NFPA) lists the MESGfor hydrogen as 0.28 mm. Thus, in one embodiment the diameter of thefluid conduction elements range from about 0.05 mm to about 0.43 mm. Thelength of the fluid conduction elements can also be restricted such thatthe length is greater than about 150% of the diameter. These values arederived from experimental work such as that described in Britton, L. G.,“Using maximum experimental safe gap to select flame arresters”, ProcessSafety Progress, 19, 140-145 (2004).

When these geometric design requirements relating to conduit sizing areused to fabricate a device and a reduced pressure insulating volume isalso incorporated, it becomes unlikely for a flame to propagate outsideof the apparatus during a device failure event. Specifically, in theinsulating volume, the reduced pressure/low oxygen level blockspropagation. In the fluid connections, the small diameter and sufficientlength restricted geometries block combustion propagation. It issensible to use the combustion geometry values for hydrogen because itis the most conservative case. However, more accurate values for the gasmixture present in the device are also anticipated. For example, theNFPA lists the MESG for butane at 1.07 mm.

Embodiments using the MESG for butane may have fluid conduction elementsthat range in diameter from about 0.05 mm to about 1.65 mm. If theseapproaches are combined with the other safety features listed above,many safe user friendly device embodiments are possible.

Additional Exemplary Device Implementations

The safety, control, monitoring, and authenticating methods and featuresdiscussed above can be incorporated in various exemplary systems anddevices. Thus, e.g., different device implementations can incorporatethe sensors, control systems, authentication circuits, and otherfeatures. Some exemplary device implementations that can further includethe safety features discussed above are provided below.

In a first exemplary device implementation, the device relates to a fuelcell apparatus that includes a housing. The housing defines asubstantially isothermal zone. As such, the housing integrates a fuelcell and a tail gas burner with the isothermal zone. The fuel cell andthe tail gas burner are in thermal communication and share a commonwall.

In a second exemplary device implementation, a housing integrates a fuelreformer and the fuel reformer is in thermal communication with the fuelcell. The fuel cell and the tail gas burner are arranged to produce apower density greater than or equal to about 2 W/cc. The fuel cell is asolid oxide fuel cell. Alternatively, the solid oxide fuel cell includesa membrane layer having a thickness less than or equal to about 500 μm,about 1 mm, or about 1.5 mm in other embodiments. The solid oxide fuelcell can include a plurality of fuel cells defining a plane therebycreating an in-plane fuel cell stack. While in another implementation,the housing includes two in-plane fuel cell stacks that aresubstantially parallel.

Different device implementations can include a low thermal conductancefluid connection element in fluid communication with the tail gasburner. In another embodiment, the low thermal conductance fluidconnection element is a micromachined fluid conducting tube, aconcentric tube, or a glass capillary tube. A low thermal conductanceelectrical element in electrical communication with the fuel cell isincluded in some embodiments of the invention. The low thermalconductance electrical element has a diameter less than or equal toabout 50 μm in one embodiment. Alternatively, an insulating volume isdisposed adjacent to an exterior of the housing in one embodiment. Fordevices having an insulating volume, the volume can include a reducedpressure, an insulating foam, a thermal reflector, or combinationsthereof. One implementation further includes a heat recuperator inthermal communication with the fuel gas burner. Additionally, the heatrecuperator can be located in the insulating volume.

In some implementations, the fuel reformer converts complex fuels suchas butane into smaller molecules for more efficient utilization by thefuel cell membrane. In some aspects the terms fuel reformer and fuelprocessor can be used interchangeably as known to those in the art.Additionally, in some aspects, the terms tail gas burner and catalyticconverter can be used interchangeably as known to those in the art. Insome aspects and embodiments, the tail gas burner burns and extractsuseful heat from any fuel in the exhaust stream not already converted orconsumed by the fuel cell. In some aspects and embodiments, the heatrecuperator or heat exchanger extracts thermal energy from the exhaustflow of the reactor for use in pre-heating the incoming fuel and airstreams for the fuel cell.

A third exemplary device implementation, relates to a fuel cellapparatus that includes a fuel cell and a tail gas burner in thermalcommunication with the fuel cell. The fuel cell and the tail gas burnerre arranged to produce a power density greater than or equal to about 2W/cc.

A fourth exemplary device implementation, relates to a method ofminimizing heat loss during operation of a solid oxide fuel cell. Themethod includes the steps of providing a housing containing a fuel cell,and operating the fuel cell so the ratio of power to the volume of thehousing is greater than about 2 W/cc.

In fifth exemplary device implementation, the device relates to a fuelcell apparatus that includes a first solid oxide fuel cell and a secondsolid oxide fuel cell. The first solid oxide fuel cell includes an anodelayer, a cathode layer, and an electrolyte layer. In turn, the secondsolid oxide fuel cell includes an anode layer, a cathode layer, and anelectrolyte layer. In this aspect, the distance between a center line ofthe electrolyte layer of the first solid oxide fuel cell and a centerline of the electrolyte layer of the second solid oxide fuel cell isless than or equal to about 1.5 mm or about 1 mm.

In a sixth exemplary device implementation, the device relates to a fuelcell apparatus that includes a solid oxide fuel cell and a low thermalconductance fluid connection element in fluid communication with thesolid oxide fuel cell. The solid oxide fuel cell is adapted to operateat a temperature greater than or equal to about 400° C. Also, the lowthermal conductance fluid connection element is designed to produce athermal loss due to the solid cross section of the low thermalconductance fluid connection element such that the loss is less thanabout 0.1 watts per low thermal conductance fluid connection element.

In a seventh exemplary device implementation, the device relates to anapparatus that includes a solid oxide fuel cell and a low thermalconductance electrical element in electrical communication with thesolid oxide fuel cell. The solid oxide fuel cell is adapted to operateat a temperature greater than or equal to about 600° C. and the lowthermal conductance electrical element has a resistance greater than orequal to about 0.5 ohms. In one embodiment, the low thermal conductanceelectrical element includes platinum and has a diameter less than orequal to about 200 μm or about 100 μm.

In an eighth exemplary device implementation, the device includes ahousing containing a solid oxide fuel cell and an insulating volumedisposed adjacent to an exterior of the housing. The insulating volumeis at a reduced pressure.

In a ninth exemplary device implementation, the device includes ahousing containing a solid oxide fuel cell, an insulating volumedisposed adjacent to an exterior of the housing, and a heat exchanger inthermal communication with the solid oxide fuel cell. The heat exchangeris located in the insulating volume.

In a tenth exemplary device implementation, the device includes a solidoxide fuel cell and a low thermal conductance fluid connection elementin fluid communication with the solid oxide fuel cell. The solid oxidefuel cell is adapted to operate at a temperature greater than or equalto about 400° C. The low thermal conductance fluid connection element isdesigned to produce a thermal loss due to its solid cross section thatis less than about 0.1 watts per low thermal conductance fluidconnection element.

In an eleventh exemplary device implementation, the device includes asolid oxide fuel cell and a low thermal conductance electrical elementin electrical communication with the solid oxide fuel cell. The solidoxide fuel cell is adapted to operate at a temperature greater than orequal to about 400° C. and the low thermal conductance electricalelement has a resistance greater than or equal to about 0.5 ohms. Incertain embodiments, the low thermal conductance electrical elementcomprises platinum and/or has a diameter less than or equal to about 200μm.

In a twelfth exemplary device implementation, the device relates to afuel cell apparatus that includes a housing containing a solid oxidefuel cell and an insulating volume disposed adjacent to an exterior ofthe housing. The insulating volume is at a reduced pressure.

In a thirteenth exemplary device implementation, the device relates to afuel cell apparatus that includes a housing containing a solid oxidefuel cell, an insulating volume disposed adjacent to an exterior of thehousing, and a heat exchanger in thermal communication with the solidoxide fuel cell. The heat exchanger is located in the insulating volume.

In a fourteenth exemplary device implementation, the device relates to afuel cell apparatus that includes a space separation means for defininga substantially isothermal zone and for integrating elements togetherwithin a particular space. As such, the space separation meansintegrates a means for turning fuel into electricity and a means forburning and extracting thermal energy from any fuel within theisothermal zone. The means for turning fuel into electricity and themeans for burning and extracting thermal energy are in thermalcommunication and share a common wall. In one embodiment, the spaceseparation means is a housing. In another embodiment, the spaceseparation means is an outer wall. In yet another embodiment, the spaceseparation means is a semiconductor structure.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The foregoingembodiments are therefore to be considered in all respects illustrativerather than limiting on the invention described herein. Scope of theinvention is thus indicated by the appended claims rather than by theforegoing description, and all changes which come within the meaning andrange of equivalency of the claims are intended to be embraced therein.

Each of the patent documents and scientific publications disclosedhereinabove is incorporated by reference herein for all purposes.

1. An apparatus, comprising a fuel cell housing, a fuel cell disposedwithin the fuel cell housing, and an insulating volume disposed adjacentto an exterior of the fuel cell housing and defining a region having apressure selected to provide thermal insulation proximate the exteriorof the fuel cell housing.
 2. The apparatus of claim 1, furthercomprising a device housing placed around the fuel cell housing andspaced away from the exterior of the fuel cell housing to define achamber for the insulating volume.
 3. The apparatus of claim 1, whereinthe fuel cell housing has an interior chamber having a volume of lessthan or equal to about 100 cc.
 4. The apparatus of claim 1, wherein thereduced pressure is less than or equal to about 100 mtorr.
 5. Theapparatus of claim 1, further comprising a fuel reformer disposed withinthe fuel cell housing and in fluid communication with the fuel cell. 6.The apparatus of claim 1, further comprising a tail gas burner disposedwithin the fuel cell housing and in fluid communication with the fuelcell.
 7. The apparatus of claim 1, further comprising: a sensorpositioned to collect data; and a fluid flow controller in communicationwith the sensor.
 8. The apparatus of claim 7, wherein the sensor ispositioned in communication with the insulating volume, and the sensoris one of a pressure detector or a temperature detector.
 9. Theapparatus of claim 7, wherein the sensor is positioned in thermalcommunication with the fuel cell, and the sensor is a temperaturedetector.
 10. The apparatus of claim 7, wherein the sensor is selectedfrom the group consisting of a fluid flow detector, a chemical detector,a pressure detector, a comparator circuit, a voltage detector, a currentdetector, a direct mass flow rate detector, an indirect mass flow ratedetector, a volume flow detector, a differential detector, a temperaturedetector, a radiation detector, and combinations thereof.
 11. Theapparatus of claim 7, wherein the data collected is selected from thegroup consisting of temperature data, fluid flow data, pressure data,radiation data, electric signal data, electric current data, voltagedata, geometric data, structural stability data, vibration data, sheerstress data, chemical composition data, and combinations thereof. 12.The apparatus of claim 7, wherein the fluid flow controller is selectedfrom the group consisting of a valve, a unidirectional flow device, agasket, a seal, a gate, a membrane, an iris, an occluder, a vent, aconduit, and combinations thereof.
 13. The apparatus of claim 7, furthercomprising a fuel source in fluid communication with the fuel cell,wherein the fuel source comprises an authentication circuit incommunication with the fluid flow controller.
 14. The apparatus of claim7, further comprising a first conduit in fluid communication with thefuel cell, wherein the fluid flow controller is adapted to regulatefluid flow through the first conduit.
 15. The apparatus of claim 1further comprising a first conduit in fluid communication with a fuelreformer and a second conduit in fluid communication with a tail gasburner, and at least one sensor for measuring fluid flow in at least oneof the first or second conduit.
 16. The apparatus of claim 1, furthercomprising a mechanical support connected between the device housing andthe fuel cell housing.
 17. An apparatus, comprising: a fuel cellhousing, a fuel cell disposed within the fuel cell housing, aninsulating volume disposed adjacent to an exterior of the fuel cellhousing and defining an area of reduced thermal conduction, and a flamearrester conduit in fluid communication with the fuel cell and anexterior of the insulating volume, wherein the flame arrester conduitspans a portion of the insulating volume.
 18. The apparatus of claim 17,wherein the insulating volume includes at least one of an area ofreduced pressure, an insulating fiber material, an insulating gelmaterial, a ceramic material, and a circulating thermally conductingfluid.
 19. The apparatus of claim 17, wherein the flame arrester conduithas a diameter that is less than or equal to about 1.54 times a MaximumExperimental Safe Gap for a fluid used in the apparatus.
 20. Theapparatus of claim 17, wherein the flame arrester conduit has a diameterfrom about 0.05 mm to about 0.43 mm.
 21. The apparatus of claim 17,wherein the insulating volume defines an area of reduced pressure beingless than or equal to about 100 mtorr.
 22. The apparatus of claim 17,further comprising a control system associated with the fuel cell,wherein the control system is selected from the group consisting of afluid flow detector, a pressure detector, a comparator circuit, avoltage detector, a current detector, a direct mass flow rate detector,an indirect mass flow rate detector, a volume flow detector, adifferential detector, a feedback loop, a temperature detector, aradiation detector, a valve, a unidirectional flow device, a gasket, aseal, a gate, a membrane, an iris, an occluder, a vent, a conduit andcombinations thereof.
 23. An apparatus, comprising a fuel cell housinghaving a fuel cell disposed therein, an insulating volume disposedadjacent to an exterior of the fuel cell housing, an input air conduitin fluid communication with the fuel cell housing, an input fluiddelivery conduit in fluid communication with the fuel cell; an outputexhaust conduit in fluid communication with the fuel cell, wherein eachconduit spans a portion of the insulating volume.
 24. The apparatus ofclaim 23 wherein the input air conduit is associated with a firstsensor, the input fluid delivery conduit is associated with a secondsensor, and the output exhaust conduit is associated with a thirdsensor.
 25. The apparatus of claim 23, wherein the insulating volumedefines a region of reduced pressure.
 26. The apparatus of claim 25,wherein volume of the housing ranges from about 0.5 cc to about 100 cc.27. A method of manufacturing a fuel cell, comprising: providing a fuelcell housing having an interior chamber capable of supporting a reducedpressure and disposing a fuel cell therein, arranging the fuel cellhousing within a device housing to space an exterior wall of the fuelcell housing away from an interior wall of the device housing, andreducing a pressure within the interior chamber to define a regionhaving a pressure selected to provide thermal insulation proximate theexterior of the fuel cell housing.
 28. The method of claim 27, furthercomprising arranging within the device housing a senor for measuring aparameter representative of a pressure or a temperature in the regionhaving a selected pressure.
 29. The method of claim 27, furthercomprising providing a valve for adjusting the pressure within theinterior chamber responsive to the parameter.
 30. The method of claim27, further comprising connecting a stand-off support between the fuelcell housing and the device housing to support the fuel cell housing andto space the fuel cell housing away from the device housing.
 31. Themethod of claim 27, further comprising providing a radiation shieldbetween the fuel cell housing and an interior wall of the devicehousing.
 32. The method of claim 27, further comprising attaching afluid conduit to the fuel cell housing and the device housing fordelivering fuel to the fuel cell housing and for supporting the fuelcell housing within the device housing.